Multi-Carrier Modulated communications systems, so called MCM communications systems, have become very popular of late and are used for instance in the LTE, Long Term Evolution, cellular communication system and also in many optical communications systems. In MCM communications systems, an available communications channel is divided into frequency chunks, called sub-carriers. The resulting sub-carriers therefore have a comparably narrow bandwidth compared to the total system bandwidth.
A drawback of traditional MCM transceivers is a certain sensitivity to phase noise. This is due to the fact that phase noise causes ICI, Inter-Carrier Interference, between the sub-carries of an MCM system, and also to the fact that traditional phase compensation methods based on feedback loops, e.g., the PLL, Phase Locked Loop, are not well suited for MCM systems. The reason why phase compensation methods based on feedback loops are ill suited for use in MCM systems are mainly two-fold:
First, since the sub-carriers in an MCM system have relatively narrow bandwidths, modulated data symbols transmitted over MCM sub-carriers will be of a comparably long symbol time duration, which will limit the achievable bandwidth of a phase compensation system based on a feedback loop such as a PLL.
Second, MCM demodulation requires frequency domain techniques that always incur a delay in the time domain. This delay will further limit the achievable bandwidth of a phase compensation system based on a feedback loop such as a PLL.
In some MCM systems, frequency domain pilot symbols are often used for aiding in phase compensation, and complex methods are used for ICI mitigation to suppress the residual ICI effects of phase noise. Such means to alleviate phase noise effects add significantly to the complexity of the MCM transceiver.
U.S. Pat. No. 8,139,681 B2 describes a PLL-based frequency correction system that comprises re-modulation of a received communications signal. The update rate of the PLL loop filter is thus high, even if the receiver is used to receive an MCM signal with long symbol duration. However, the attainable bandwidth in the PLL is low due to the loop feedback delay, which thus limits the performance of the device of this document.